1. Field of the Invention
The present application relates to a driver for an electro-absorption (EA) modulator, in particular, the application relates to a driver to prevent an excess optical output when a modulation signal becomes absent.
2. Background Arts
Recent transmitter optical modules often install an external modulator to modulate light coming from an optical source as a continuous wave because the modulating frequency or the modulating speed in the recent optical communication system exceeds 25 Gbps or sometimes reaches 40 Gbps. One type of the external modulator is an EA modulator. Because an EA modulator inherently shows nonlinearity between the modulation signal and the optical output therefrom, a driver to driver the EA modulator usually provides a function to adjust the cross point of the output of the driver, namely, the input of the EA modulator.
The adjustment of the cross point of the output of the driver is carried out to superpose an offset in the output; accordingly, when the modulation signal is absent, the superposed offset possibly causes an excess optical output in the EA modulator. The present application is to provide a technique to prevent or suppress the excess optical output from the EA modulator.